1. Field of the Invention
This invention relates to an ignition apparatus for an internal combustion engine capable of preventing generation of ignition sparks during the reverse rotation of a crank shaft of the engine.
2. Description of the Prior Art
When starting an internal combustion engine, a starter is generally energized through an ignition switch to rotate a rotary member, such as a crank shaft, while ignition sparks are generated by an ignition plug once for every rotation thereof to cause the mixed gas to explode and thus to initiate and continue the engine rotation. In order to generate ignition sparks from the ignition plug, a conventional ignition apparatus is provided with a sensor operable in synchronization with the crank shaft of the engine for detecting its specific angular positions and with a microcomputer receiving the output of the sensor. The signal output from the microcomputer is supplied to a transistor to the output end of which an ignition coil is connected, so that when the sensor has detected a specific angular position of the engine crank shaft a signal is generated from the microcomputer to turn off the transistor, thereby causing an instantaneous flow of high current through the ignition coil to generate an ignition spark at the ignition plug.
FIG. 1 is a circuit diagram illustrating an example of a conventional ignition apparatus. In this figure, a sensor 1 is provided in association with an internal combustion engine EN for the purpose of detecting the angular postions of a crank shaft CS thereof. The output of the sensor 1 is connected to an input port 21 of a microcomputer 2 which is, in turn, connected at its output port 22 to the base electrode of a transistor 3. The emitter electrode of the transistor 3 is grounded and the collector electrode thereof is connected to one end of the primary winding of an ignition coil 4. The other end of the primary winding of the coil 4 is connected to the positive end of a battery 5, the negative end of which is grounded. On the other hand, the output end of the secondary winding of the coil 4 is grounded through an ignition plug 6.
A practical example of the sensor 1 is shown in FIG. 2. A projection 12 is formed on an outer peripheral portion of a rotor 11 which is connected to the engine crank shaft CS for rotation therewith in the direction shown by an arrow X. The projection 12 extends between first and second angular positions .theta..sub.1 and .theta..sub.2 of the crank shaft CS. Located adjacent to the outer periphery of the rotor 11 is a proximity switch 13 which generates between its output terminal 14 and a ground terminal 15 a high level output during the period when the projection 12 is opposed to the switch and also a low level output during the remaining period. Thus, as the rotor 11 rotates in the direction shown by the arrow X the output level of the output terminal 14 is turned from a low level to a high level (first state) at the first angular position .theta..sub.1, as shown in FIG. 3, and returns from the high level to the low level (second state) at the second angular position .theta..sub.2.
FIG. 4 illustrates waveforms of the output voltage a of the sensor 1, the output voltage b of the microcomputer 2, the collector voltage c of the transistor 3 and the secondary output voltage d of the ignition coil 4. The operation of the ignition apparatus of FIG. 1 will be described below with reference to the illustration in FIG. 4. It is assumed that the ignition switch has been turned ON to drive the starter and the crank shaft CS and thus that the rotor 11 has been rotated in the direction shown by the arrow X in FIG. 2, such a rotation of the rotor 11 being hereafter referred to as the forward rotation and the rotation thereof in the direction opposite to the arrow X being referred to as the reverse rotation. In FIG. 4 rotor 11 is assumed to make the forward rotation during the period up to the time t.sub.8 and the reverse rotation after that time. At time t.sub.1, when the sensor 1 detects the first angular position .theta..sub.1 of the rotor 11, the logic level of the input port 21 of the microcomputer 2 turns from low to high to cause the microcomputer 2 to compute the time that has elapsed from the preceding rise of the level of the input port 21, thereby allowing a predicted ignition time t.sub.2 to be calculated by the microcomputer 2. As a result, the output port 22 of the microcomputer 2 turns to the low level at the time t.sub.2 after the lapse of a predetermined time from the time t.sub.1, and the transistor 3 turns OFF to cut off the current flowing through the primary winding of the ignition coil 4, thereby generating an ignition spark at the ignition plug 5. At time t.sub.3, when the sensor 1 detects the second angular position .theta..sub.2, the input port 21 of the microcomputer 2 turns to low level and the microcomputer 2 calculates a predicted time t.sub.4 for initiating the flow of current through the ignition coil 6 based on the calculation of the time that has elapsed from the preceding rise of the level of the input port 21 to the time t.sub.1. Thus, the output port 22 of the microcomputer 2 turns to the high level at the time t.sub.4 after the lapse of a predetermined time from the time t.sub.3 and the transistor 3 turns ON to cause the current to flow through the primary winding of the ignition coil 6. At a time t.sub.5, when the sensor 1 detects the first angular position .theta..sub.1, the level of the input port 21 of the microcomputer 2 again turns from low to high, allowing the time period from the time t.sub.1 to the time t.sub.5 to be calculated, thereby determining a subsequent predicted ignition time t.sub. 6 with the aid of the microcomputer 2. Thus, at a time t.sub.6 after the lapse of a predetermined time from the time t.sub.5, the output port 22 of the microcomputer 2 falls to the low level and the transistor 3 turns OFF to cut off the current flowing through the primary winding of the ignition coil 4, thereby generating an ignition spark on the ignition plug 5. Assuming that, after the sensor 1 has detected the second angular position .theta..sub.2 at a time t.sub.7, the engine starts to reversely rotate at a time t.sub.8, the sensor 1 detects the second angular position .theta..sub.2 at a time t.sub.10, the output thereof is turned from high to low, the first angular position .theta..sub.1 is detected at a time t.sub.12 and the output of the sensor 1 again turns from high to low. The microcomputer 2 predicts the time at which the level of the output port 22 changes in response to the change in level of the input signal of the input port 21, so that at a time t.sub.9 after the lapse of a predetermined time from the time t.sub.7, the output port 22 is turned to the high level to turn ON the transistor 3 and causes the current to flow through the primary winding of the ignition coil 4. A predicted ignition time t.sub.11 is determined as a result of the computed time period from the time t.sub.5 to the time t.sub.10 in response to the change in level of the input port 21 from low to high at the time t.sub.10. At the time t.sub.11 after the lapse of a predetermined time from the time t.sub.10, the output port 22 then falls to the low level to turn OFF the transistor 3 and cut off the primary current of the ignition coil 4. Consequently, an ignition spark will be generated at the ignition plug 5 at a wrong or incorrect ignition position.
As described above, the conventional ignition apparatus encounters problems in that generation of an ignition spark at wrong or incorrect time may occur during the reverse rotation of the engine such as to cause damage.